It should be understood that the invention can be used generally for imparting etch patterns on a metal substrate using an etch resistant mask. However, the invention shall be particularly described with reference to its use in making flexible printed circuit boards having metal substrates.
In both additive and substractive techniques for printed circuit manufacture, a great variety of base materials has been employed as a substrate material. One of the substrates of interest utilizes a thin metal foil such as a 1 to 7 mil steel foil which is coated with a flexible dielectric material. Circuit patterns are then formed over the flexible circuit board by any of the well known coating techniques. In order to be suitable for use as a printed circuit board, the metal foil must first be processed so as to provide a plurality of spaced through-holes therein. The metal foil containing the through-holes may then be coated by any of the known coating techniques with an appropriate polymer coating. For example, one may use coating techniques and compositions as set forth in the patent issued to J. J. Chang, U.S. Pat. No. 4,107,837. Alternatively, one may refer to patents issued to C. A. McPherson, U.S. Pat. No. 4,121,015 and R. B. Lewis and T. A. Giversen, U.S. Pat. No. 4,176,142 which provide alternate coating formulations suitable for providing flexible printed circuit boards using thin metal foil substrates. The teachings of all of the above patents are incorporated herein by reference.
One of the problems in manufacturing the flexible printed circuit board is that of providing sharp, well defined, through-holes. Previous solutions to the problem of providing through-holes by means of selective chemical milling or etching have involved techniques such as printing an image on the metal with an etch resistant ink followed by chemical etching; or photoimaging a photosensitive resist that is spread on the metal substrate, imaged, cured and developed, and then, etching the remaining exposed areas of the metal substrate; or laminating photosensitive film onto the metal and then light imaging an etch mask pattern thereon followed by etching. When printing with an etch resistant ink, or using a photoresist, there are often problems due to the presence of pin-holes in the ink or resist. Furthermore, the coatings must be self-adhesive on the substrate, even under the vigorous spray action of liquid etchant and must also be strippable after the sample is etched. Each of the above mentioned methods have often been found to be less than adequate with regard to either its adhesiveness or its ability to be stripped. Still another factor is that none of the prior art etch mask coatings or films are reusable. If one attempts to use a metal mask which has been coated so as to be etch resistant, great difficulty is encountered in placing the mask on the substrate to be etched so as to prevent undercutting and seepage of the liquid etchant between the mask and the substrate thereby etching areas of the substrate which were to remain unetched. Such masks made from or containing a magnetic metal and held in place on the substrate by a magnet have been described in U.S. Pat. Nos. 3,002,847 and 3,170,810.
It would, therefore, be desirable to provide a method whereby a reusable mask, particularly a ferro-magnetic mask, can be employed which adheres well to the substrate so as to prevent seepage of the etchant between the mask and the substrate and yet is easily removed from the substrate subsequent to etching.